Patients with the Smith-Lemli-Opitz syndrome have a cholesterol deficiency because there is a block in the synthesis of cholesterol. Clinically, they have microcephaly, mental retardation and hypotonicity, along with very low plasma cholesterol levels and high plasma concentrations of 7- dehydrocholesterol This study will attempt to identify a biomarker for the carrier state in parents of Smith-Lemli-Opitz patients by stimulating cholesterol biosynthesis by feeding a cholesterol-free diet and administering bile acid binding resins.